In recent years, against a background of environmental problems, secondary batteries typified by lithium-ion batteries have been applied not only to moving objects such as hybrid electric vehicles (HEV), electric vehicles (EV), forklifts, and loading shovels but also to industrial uses such as uninterruptible power systems (UPS) and power storage for photovoltaic power generation. With the expansion of the uses of the secondary batteries, an increase in the capacity as well as an increase in the energy density is demanded. In addition to increasing the performance, increasing the safety is also a significant challenge to be addressed. Among the secondary batteries are nickel-cadmium batteries, nickel-hydrogen batteries, and lithium-ion batteries. As cadmium is toxic, the nickel-cadmium batteries have been replaced with nickel-hydrogen batteries and lithium-ion batteries. Among all the currently available secondary batteries, lithium-ion secondary batteries are particularly suitable for increasing the energy density, and thus are actively developed even now.
The main components of a nickel-hydrogen battery and a lithium-ion secondary battery are a metallic current collector (a negative electrode) having a negative electrode active material layer formed on a surface thereof, a separator that retains an electrolyte, and another metallic current collector (a positive electrode) having a positive electrode active material layer formed on a surface thereof. In regard to a nickel-hydrogen battery, the positive electrode contains nickel oxide, and the negative electrode contains a hydrogen storage alloy. In regard to a lithium-ion secondary battery, the positive electrode contains lithium-metal oxide, and the negative electrode contains a carbon material such as graphite.
The battery structures are broadly divided into a wound structure in which a belt-like negative electrode, separators, and a positive electrode are wound in a spiral fashion, and a stacked structure in which strip-like negative electrodes, separators, and positive electrodes are alternately arranged. In the case of a wound, hermetically sealed battery with the wound structure, gas generated in the battery can should be promptly and smoothly guided to a portion around a gas exhaust valve, while how to arrange lead pieces for electrically connecting the belt-like negative electrode or positive electrode group to an electrode pole, which connects to a battery terminal, is a factor to decide the passage for the gas.
Among the gas exhaust mechanisms of the wound, hermetically sealed batteries is the invention disclosed in Patent Literature 1. Patent Literature 1 discloses that, when a plurality of lead pieces exist in a manner overlapping each other, between an end face of a wound electrode plate group and a battery cap, which is located opposite thereto and has a gas exhaust valve, a failure that a gas passage may be blocked is avoided by forming a gas passage by shifting the phase of the arrangement of the lead pieces on the electrode plate in winding the electrode plate such that the arrangement is not at regular intervals and thus intentionally creating a gap. Further, there may also be cases where a gas passage is provided by providing holes in the electrode pole arranged on the exhaust passage for the gas.